1. Field of Invention
The invention relates to methods for preparing single walled carbon nanotubes which are uniform in diameter. More specifically, the invention relates to methods for preparing single walled carbon nanotubes from a metal catalyst which has been seeded or coated with fullerene.
2. Carbon Nanotubes
This invention lies in the field of carbon nanotubes (also known as fibrils). Carbon nanotubes are vermicular carbon deposits having diameters less than 1.0μ, preferably less than 0.5μ, and even more preferably less than 0.2μ. Carbon nanotubes can be either multi walled (i.e., have more than one graphite layer on the nanotube axis) or single walled (i.e., have only a single graphite layer on the nanotube axis). Other types of carbon nanotubes are also known, such as fishbone fibrils (e.g., resembling nested cones), etc. As produced, carbon nanotubes may be in the form of discrete nanotubes, aggregates of nanotubes (i.e., dense, microscopic particulate structure comprising entangled or bundled carbon nanotubes) or a mixture of both.
Carbon nanotubes are distinguishable from commercially available continuous carbon fibers. For instance, the diameter of continuous carbon fibers, which is always greater than 1.0μ and typically 5 to 7μ, is far larger than that of carbon nanotubes, which is usually less than 1.0μ. Carbon nanotubes also have vastly superior strength and conductivity than carbon fibers.
Carbon nanotubes also differ physically and chemically from other forms of carbon such as standard graphite and carbon black. Standard graphite, because of its structure, can undergo oxidation to almost complete saturation. Moreover, carbon black is an amorphous carbon generally in the form of spheroidal particles having a graphene structure, such as carbon layers around a disordered nucleus. On the other hand, carbon nanotubes have one or more layers of ordered graphenic carbon atoms disposed substantially concentrically about the cylindrical axis of the nanotube. These differences, among others, make graphite and carbon black poor predictors of carbon nanotube chemistry.
Multi walled and single walled carbon nanotubes differ from each other. For example, multi walled carbon nanotubes have multiple layers of graphite along the nanotube axis while single walled carbon nanotubes only have a single graphitic layer on the nanotube axis.
The methods of producing multi walled carbon nanotubes also differ from the methods used to produce single walled carbon nanotubes. Specifically, different combinations of catalysts, catalyst supports, raw materials and reaction conditions are required to yield multi walled versus single walled carbon nanotubes. Certain combinations will also yield a mixture of multi walled and single walled carbon nanotubes.
Processes for forming multi walled carbon nanotubes are well known. E.g., Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83; Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993); Oberlin, A. and Endo, M., J. of Crystal Growth, Vol. 32 (1976), pp. 335-349; U.S. Pat. No. 4,663,230 to Tennent et al.; U.S. Pat. No. 5,171,560 to Tennent et al.; Iijima, Nature 354, 56, 1991; Weaver, Science 265, 1994; de Heer, Walt A., “Nanotubes and the Pursuit of Applications,” MRS Bulletin, April, 2004; etc. All of these references are herein incorporated by reference.
Processes for making single walled carbon nanotubes are also known. E.g., “Single-shell carbon nanotubes of 1-nm diameter”, S Iijima and T Ichihashi Nature, vol. 363, p. 603 (1993); “Cobalt-catalysed growth of carbon nanotubes with single-atomic-layer walls,” D S Bethune, C H Kiang, M S DeVries, G Gorman, R Savoy and R Beyers Nature, vol. 363, p. 605 (1993); U.S. Pat. No. 5,424,054 to Bethune et al.; Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Lett. 243: 1-12 (1995); Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E., Science, 273: 483-487 (1996); Dai., H., Rinzler, A. G., Nikolaev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Lett. 260: 471-475 (1996); U.S. Pat. No. 6,761,870 (also WO 00/26138) to Smalley, et. al; “Controlled production of single-wall carbon nanotubes by catalytic decomposition of CO on bimetallic Co—Mo catalysts,” Chemical Physics Letters, 317 (2000) 497-503; Maruyama, et. al. “Low-temperature synthesis of high-purity single walled carbon nanotubes from alcohol,” Chemical Physics Letters, 360, pp. 229-234 (Jul. 10, 2002); U.S. Pat. No. 6,333,016 to Resasco, et. al.; R. E. Morjan et al., Applied Physics A, 78, 253-261 (2004), etc. All of these references are hereby by reference.
Additionally, Maruyama, S., “Morphology and chemical state of Co—Mo catalysts for growth of single-walled carbon nanotubes vertically aligned on quartz substrates,” Journal of Catalysis, 225, pp. 230-239 (2004), described a method of growing single walled nanotube forest on a flat surface under vacuum. A bimetallic catalyst containing Co and Mo precursor was first deposited on a quartz surface followed by calcination and reduction to form highly dense-packed metal particles. The growth of single-walled carbon nanotubes from these metal particles presented a density of 1×1017/m2 with length of approximately 5 micrometers. K. Hata, “Water-assisted highly efficient synthesis of impurity-free single-walled carbon nanotubes,” Science, 306, pp. 1362-1364 (2004), described another technique using water-assisted CVD method to grow single-walled carbon nanotube forest from a Si wafer coated with iron thin film. They observed water-stimulated enhanced catalytic activity results in massive growth of superdense (1014-1015/m2) and vertically aligned nanotube forests with heights up to 2.5 millimeters. All of these references are hereby by reference.
Other known processes include WO 2006/130150, “Functionalized Single Walled Carbon Nanotubes” and U.S. Pat. No. 6,221,330, “Process For Producing Single Wall Nanotubes Using Unsupported Metal Catalysts And Single Wall Nanotubes Produced According To This Method” Additionally, in “Synthesis of single-walled carbon nanotubes with narrow diameter-distribution from fullerene,” Chem. Phys. Lett., 375, pp. 553-559 (2003), Maruyama et al. reported using alcohol as carbon source to grow single-walled carbon nanotubes at relative low temperatures, e.g. 550-800° C. The diameter distribution of those as-grown single-walled nanotubes was found to be very broad (0.8-1.3 nm) and uniformity was poor and temperature dependent. When fullerene was directly applied as carbon source, the authors found some improvement of diameter distribution to 0.8-1.1 nm, but the uniformity was still unclear according to the Raman spectroscopy. All of these references are hereby incorporated by reference.
However, currently known single walled carbon nanotube processes tend to yield a wide distribution of single walled carbon nanotube sizes. Measurements of diameters of single walled carbon nanotubes are usually done using Raman spectrometry. A typical Raman spectrometer equipped with continuous He—Ne laser with wavelength of 632.8 nm is used to collect Raman excitation. A Raman peak at ˜1580 cm−1 is present in all types of graphite samples such as highly oriented pyrolytic graphite (HOPG), pyrolytic graphite and charcoal. This peak is commonly referred to as the ‘G-band’. The peak at 1355 cm−1 occurs when the material contains defects in the graphene planes or from the edges of the graphite crystal. This band is commonly referred to as the ‘D-band’ and the position of this band has been shown to depend strongly on the laser excitation wavelength. “Radial breathing modes (RBM)” (typically below 300 cm−1) were observed with single-walled nanotubes, where all the carbon atoms undergo an equal radial displacement. A small change in laser excitation frequency produces a resonant Raman effect. Investigation in the RBM has shown it to be inversely proportional to the SWCNT diameter. This relationship is expressed in the following equation,ωRBM=(223.75/d)cm−1 where ωRBM is the RBM frequency, and d is the SWCNT diameter (in nanometers). The relationship is slightly different for determining individual nanotubes. Bandow, et al. “Effect of the growth temperature on the diameter distribution and chirality of single-wall carbon nanotubes,” Physical Review Letters, 80, pp. 3779-3782 (1998), Jishi, et al. “Phonon modes in carbon nanotubes,” Chemical Physics Letters, 209, pp. 77-82 (1993). All of these references are hereby incorporated by reference.
In the above equation and throughout this specification, diameter of a nanotube is defined as the distance between the nuclei of carbon atoms at opposite ends of a tube diameter. It is to be understood that this diameter differs from distance of closest approach by a second nanotube which is greater because of the repulsion of the respective π clouds as often defined by TEM.
Table A presents sample diameter and ωRBM correlations as previously reported in Tables I and II of Jorio, A, et al., “Structural (n,m) Determination of Isolated Single-Wall Carbon Nanotubes by Resonant Raman Scattering,” Physical Review Letters, The American Physical Society, Vol. 86, No. 6, pp. 1118-21 (Feb. 5, 2001), herein incorporated by reference:
TABLE AωRBMωRBMdtΘ(calc)(expt.)(n, m)[nm][deg][cm−1][cm−1](18, 6)1.7213.9144.4144(2)(19, 4)1.699.4146.8. . .(20, 2)1.674.7148.3. . .(21, 0)1.670.0148.8148(5)(15, 9)1.6721.8148.8. . .(12, 12)1.6530.0150.3151(3)(16, 7)1.6217.3153.0154(5)(17, 5)1.5912.5156.4156(6)(13, 10)1.5925.7156.4156(1)(18, 3)1.567.6158.8158(1)(19, 1)1.552.5160.0160(3)(14, 8)1.5321.1162.0. . .(11, 11)1.5130.0164.0164(1)(15, 6)1.4916.1166.7165(1)(16, 4)1.4610.9170.4169(1)(17, 2)1.445.5172.7174(1)(18, 0)1.430.0173.5176(1)(14, 1)1.153.4215.1210(1)(10, 6)1.1121.8223.1. . .(9, 7)1.1025.9224.9. . .(11, 4)1.0714.9232.2229(1)(10, 5)1.0519.1236.1237(2)(12, 2)1.047.6238.2. . .(8, 7)1.0327.8240.3239(2)(11, 3)1.0111.7244.7. . .
As the number of complex technical applications for carbon nanotubes increase, there is a need for an improved method for producing single walled carbon nanotubes with a more narrow size or diameter distribution so as to allow for a more precise application of single walled carbon nanotubes.